1. Field of the Invention
This invention relates to apparatus for exposing an object to collimated x-ray radiation and providing further collimation so as to permit progressive exposure of the object to a limited quantity of x-ray radiation so as to provide improved x-ray image quality and/or a reduction in radiation exposure.
2. Description of the Prior Art
In the radiographic examination of various types of objects, including medical examination of parts of the human body, by the film-screen technique, the x-ray image quality tends to be reduced by the presence of scattered x-rays in the image. This problem is of particular concern in connection with the examination of certain anatomical structures, such as the female breast, for example, because the object contrast is normally very low, and any further reduction of this low contrast, because of scattered radiation, would result in the decreased ability to diagnose tumors, such as breast cancers, early in a stage when successful therapy is most likely to be available.
There is the further problem in connection with human examinations that it is undesirable to expose living human beings to any quantities of radiation beyond those reasonably necessary for diagnostic procedures. Even where such uses are desirable, as in attempts to detect the presence of female breast tumors, the frequency with which such examinations may be performed is somewhat limited as a result of a need to use an x-ray exposure sufficiently high so as to achieve an adequate quality of image. Efforts to reduce the exposure employed in such tests tend to move away from the desirable objective of increased contrast sensitivity in the resultant image.
It has previously been suggested to use various forms of collimators in connection with x-ray apparatus in order to better define the region which will be exposed to x-ray radiation. See, for example, U.S. Pat. Nos. 3,767,931; 3,829,701; 3,934,151; 3,947,689 and 3,973,127. These disclosures, however, do not provide synchronous movement of the pair of collimators which serve to severely restrict both the amount of x-ray impinging upon the object and the x-ray emerging from the object which is permitted to impinge upon the x-ray detector, such as the x-ray film or electronic sensors.
It has previously been suggested to employ a single mechanically moved slit collimator interposed between x-ray tube and patient with a phosphorus screen and associated camera lens, TV camera and cathode-ray tube display to provide for an image which may be recorded by means of a regular film camera. See Volume 78, Society of Photo-Optical Instrumentation Engineers, pages 108-112 ("Isocon Imaging for X-Ray Diagnostics") and Volume 70, Society of Photo-Optical Instrumentation Engineers, pages 384-392 ("An Evaluation of Low Light Level Television for Breast Cancer Detection"). In both of these systems, there is lacking any second slit collimator interposed between the test object and the x-ray detector means.
It has previously been known to employ an overlying collimator in combination with a member having scatter eliminating slots beneath a patient. See "Reduction of Scatter in Diagnostic Radiology by Means of a Scanning Multiple Slit Assembly" by Barnes, Cleare and Brezovich, pages 691-694, September 1976, Radiology. It has also been known to provide a pair of moving slotted collimators for minimizing the effects of scattered radiation. See "Investigations of Moving-Slit Radiography" by Sorenson and Nelson, pages 705-711, September 1976, Radiology and "A Method to Absorb Scattered Radiation without Attenuation of the Primary Beam" by Moore, Korbuly and Amplatz, pages 713-717, September 1976, Radiology.
None of these disclosures suggests the use of a pair of slit collimators in combination with independent object supporting and compressing means for compressing the object to a more uniform thickness to minimize the amount of thickness through which the x-ray must travel nor the beneficial combination of stationary supporting and compressing means used in combination with a pair of slit moving collimators. There is also lacking any disclosure of the beneficial combination of these elements with highly sensitive x-ray detector means, such as self-scanning arrays of photodiodes optically coupled to scintillator means or film-screen cassettes.
There remains, therefore, a substantial need for a means for improving the contrast sensitivity of x-ray techniques while minimizing the quantity of x-ray to which the object is exposed. This need is particularly significant in connection with radiography of patients, as in mammography.